When query plans go wrong

SQL

Simon Sabin Independent SQL Server Consultant and Trainer Database design and development, Business Intelligence, Performance tuning and troubleshooting SQL Server since 6.5 Email: Simon@onarc.com Blog: http://Sqlblogcasts.com/blogs/simons Twitter: simon_sabin

Car crash

Works on my computer

Parameter Sniffing @

SQL tries to be clever

A bit like

Statistics

So what are the root causes

Out of date statistics Best Before: 1/4/1999

Skewed data

Multi purpose queries

Solutions Selective code paths WITH RECOMPILE OPTIMIZE FOR PLAN guides Careful as SP is compiled as one batch WITH RECOMPILE Compilation hit, plan cache bloat OPTIMIZE FOR Results in a consistent plan PLAN guides

Update Statistics Update Statistics Trace flag 2388, 2389 and 2390 Can be a performance hit Trace flag 2388, 2389 and 2390

Overview    Out of date statistics Compilation Best Query Skewed Data Supportable Multiple Code paths   Update statistics  OPTIMZE FOR WITH RECOMPILE Trace Flags

Summary You will only know if you monitor Baseline your system Identify changes in read, writes and cpu Not duration Consider the options for your situation You can win this battle

Please fill in feedback forms Q&A Now - Just ask Afterwards – I’ll be around Much Later Simon@SQLKnowHow.com @simon_sabin http://sqlblogcasts.com/blogs/simonsabin Please fill in feedback forms